The present patent application relates to the formation of a variety of visual renderings of an object data set. Such an object data set relates to actual data of an object to be examined. For example, the object is a patient to be medically examined. The object data set contains, for example, density values, a temperature distribution, the local magnetization or flow data. Other applications are also feasible, for example, applications involving hydrodynamic data or meteorological data, such as flow data, or seismic or medical diagnostic density values.
The invention relates to a rendering system with a rendering unit and a data processing unit for the processing of a multi-dimensional object data set, wherein
the object data set associates data values with positions in a multi-dimensional space, and
the data processing unit is arranged
to sample a surface model in the multi-dimensional object data set, which surface model contains positions on a predetermined surface in the multi-dimensional space.
A rendering system of this kind is known from U.S. Pat. No. 5,782,762.
The known rendering system is designed especially for visualizing the inner side of a hollow lumen, such as the intestine of a patient to be examined, on the basis of computed tomography data. The known system forms a volume file of data values by stacking a series of computed tomography images, each of which relates to a cross-section of the patient to be examined. The volume file is a data set which associates the tomographic data values with positions in the three-dimensional geometrical space. The known rendering system segments a region of interest from the volume file on the basis of comparison of the data values with a predetermined threshold value. In the region of interest an isosurface of the hollow lumen is determined, said isosurface being subjected to rendering, thus forming a view of the wall of the hollow lumen from inside the hollow lumen. Such an isosurface is a two-dimensional surface in the three-dimensional space of positions having each time the same data values. The known rendering system utilizes a so-called xe2x80x9cwireframe modelxe2x80x9d of the isosurface so as to derive the rendering of the inner wall of the hollow lumen from the volume file. The known rendering system is alternatively capable of forming a coarse rendering of the inner wall by shading or coloring the polygons of the wireframe model. This coarse rendering is used, for example in the case of a quickly changing point of view from within the hollow lumen, thus creating the impression that the observer moves inside the hollow lumen while looking around at the interior wall.
The cited United States patent also deals with increasing the speed at which rendering is executed. To this end, in conformity with the cited document it is necessary to reduce the volume file. As alternatives for such a reduction the article mentions a reduction of the number of bits/pixel in the computed tomography images from 16 bits/pixel to 8 bits/pixel, a selection of a smaller sub-volume from the volume file and a reduction of the spatial resolution of the volume file, composed of 500 computed tomography images, from 512xc3x97512xc3x97500 voxels to 256xc3x97256xc3x97500 voxels. It is also proposed to reduce the sampling of the computed tomography images.
When the known rendering system increases the speed at which the inside of the hollow lumen is rendered from successive points of view, the diagnostic quality of the image is degraded. The diagnostic quality is degraded notably because use is made of the wireframe module with the polygons shaded therein and because a significant reduction of the volume file is required. Moreover, the operation of the known rendering system is rather laborious, because a new isosurface is derived for each new view of the interior wall of the hollow lumen from a given direction.
It is an object of the invention to provide a rendering system that enables rendering of the object data set with a high diagnostic quality from different directions.
This object is achieved by means of a rendering system which is arranged in accordance with the invention so as to
derive successive representations of the surface model while viewing from respective directions of view in a range between a starting direction and a final direction,
successively render on the rendering unit the successive representations of the surface model from the respective directions of view,
derive a representation of the multi-dimensional object data set while viewing from the final direction, and subsequently
to render on the rendering unit the representation of the multi-dimensional object data set while viewing from the final direction.
In conformity with the invention the rendering system samples a surface model of the object data set that can be used for a plurality of renderings. The surface model represents a part of the object data set on a surface in the multi-dimensional space. The surface model in practice represents a surface of the object to be examined. For example, the surface model is an isosurface in the object data set. The rendering system successively shows renderings of the surface model as viewed from different directions. These renderings of the surface model, for example, are checked by the user who utilizes a control member such as a mouse. The user can thus rotate and/or shift the surface model as if it were on the rendering unit. The surface model comprises a quantity of data values which is much smaller than that of the multi-dimensional data set. This is because the surface model concerns a less-dimensional space in comparison with the object data set. Consequently, a next rendering can be very quickly ready for display on the rendering unit. This can even take place at approximately the same speed as that at which the user issues a command to render from each time a new direction of view. In practice the user can issue new commands very quickly, for example by moving the mouse, each position of the mouse corresponding to a new direction of view wherefrom the surface model is to be rendered. It is thus avoided that the rendering annoyingly lags the commands issued by the user. For the last direction of view entered by the user, that is, the final direction, the rendering system provides a representation of the (complete) object data set, so not only of the surface model. For example, the final direction is determined by the position where the user keeps the control member, such as the mouse, still. For example, the user can indicate a section through the object data set or given positions in the object data set in the representation of the object data set from the final direction. The invention makes it very easy for the user to rotate or shift the rendering of the object data set merely by showing the surface model in a different orientation. As soon as the user indicates the final direction, the complete object data set is automatically rendered in the orientation from the final direction.
These and other aspects of the invention will be described in detail with reference to the following embodiments that are defined in the dependent claims.
A suitable representation of the surface model consists of a collection of polygons having vertices formed by the positions on the relevant surface in the multi-dimensional space. A polygon model of this kind requires only few data for adequate representation of the surface. Moreover, such a polygon model can be readily manipulated by the data processing unit.
An advantageous embodiment of the rendering system selects the object data set from a larger multi-dimensional basic data set. This embodiment is suitable, for example for use in contrast angiography. It makes the rendering system in accordance with the invention particularly suitable for use as a medical diagnostic workstation. In contrast angiography computed tomography or rotational radiology is used to form profiles or projection images of a part of the patient to be examined while blood vessels are filled with a contrast agent, that is, from different projection directions. The multi-dimensional basic data set is reconstructed from said profiles or projection images. The multi-dimensional basic data set can also be reconstructed from magnetic resonance signals. The multi-dimensional data set comprises a representation of a part of the vascular system in the surrounding tissue of the patient to be examined. Segmentation enables accurate separation of the object data set from the basic data set, the object data set relating to the vascular system of the patient to be examined.
The rendering system is preferably arranged to perform the segmentation on the basis of a histogram which represents the distribution of data values in the basic data set.
This histogram very clearly shows which range of data values is suitable for the segmentation of the data values of the object data set from the basic data set. It is notably easy to select a suitable threshold value for the segmentation from the histogram. It is particularly handy to show the histogram on the rendering unit and to indicate the threshold value in the rendering of the histogram by means of the control member, for example, the mouse.
A further advantageous embodiment of the rendering system stores the surface model, for example the polygon model, in a Virtual Reality Modeling Language (VRML) format. The surface model can thus be readily exchanged between users. For example, the surface model can be observed in a web browser and it can also be dispatched by e-mail. In this way it is not necessary for different users to sample the surface model each time again.
The invention also relates to a method of processing multi-dimensional object data set. The method according to the invention is defined in claim 8. The method according to the invention enables rendering of the object data set with a high image quality from different directions and renderings from successive directions are achieved at a high rate.
The invention further relates to a computer program as claimed in claim 9. When the computer program of the invention is loaded into a general purpose computer, the computer is enabled to carry out the method of the invention as claimed in claim 8. The computer program of the invention may be supplied on a data carrier such as a CD-rom or the computer program may be downloaded via a data network such as the xe2x80x98world-wide webxe2x80x99.